Diodeless terrestrial photovoltaic solar power array

ABSTRACT

A method and device are disclosed for diodeless terrestrial photovoltaic solar power arrays. In one or more embodiments, the method and device involve a solar power array device without blocking diodes and/or without bypass diodes. The method comprises providing a solar module, a solar array tracker, a power bus, a controller, and an inverter. In one or more embodiments, the method further comprises providing a circuit breaker and/or a bi-position switch. When the controller senses that the solar module power is below a threshold level, the controller commands the solar tracker to vary the solar module&#39;s pointing until the solar module is operating at its maximum power point for the solar module&#39;s level of illumination. In some embodiments, when the controller senses that the solar module power is less than a minimum bypass threshold level, the controller commands a bi-position switch to bypass current around the solar module.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation application of, and claims thebenefit of, U.S. patent application Ser. No. 12/571,123, filed Sep. 30,2009, which is hereby incorporated by reference in its entirety.

BACKGROUND

The present disclosure relates to photovoltaic solar power arrays. Inparticular, it relates to diodeless terrestrial photovoltaic solar powerarrays.

SUMMARY

The present disclosure relates to a device, apparatus, system, andmethod for diodeless terrestrial photovoltaic solar power arrays. In oneor more embodiments, the method for providing diodeless terrestrialphotovoltaic solar power arrays involves a solar power array devicewithout blocking diodes. This method comprises providing a solar module,and connecting a solar array tracker and a circuit breaker to the solarmodule. The method further comprises connecting a power bus and acontroller to the circuit breaker, and connecting an inverter to thepower bus. Further, the method comprises sensing with the controller theoutput voltage and current of the solar module. When the controllersenses the solar module is producing power below a power thresholdlevel, the controller sends a signal to the solar array tracker to varythe solar module's pointing until the solar module is operating at itsmaximum power point for the solar module's level of illumination. And,when the controller senses that shadowing on the solar module is lessthan a minimum shadowing threshold level, the controller sends a commandto the circuit breaker to prevent current flow into the solar module,thereby preventing harm to cells of the solar module.

In one or more embodiments, the inverter is a direct current/alternatingcurrent (DC/AC) inverter. In some embodiments, the solar module containsa plurality of solar cells. In at least one embodiment, the plurality ofsolar cells are connected in series. In alternative embodiments, theplurality of solar cells are connected in parallel. In one or moreembodiments, the plurality of solar cells are connected inparallel-series combination. In at least one embodiment, the solarmodule contains only one solar cell.

In some embodiments, the method for providing diodeless terrestrialphotovoltaic solar power arrays involves a solar power array devicewithout bypass diodes. This method comprises providing a solar module,and connecting a solar array tracker and a bi-position switch to thesolar module. The method further comprises connecting a power bus and acontroller to the bi-position switch, and connecting an inverter to thepower bus. Further, the method comprises, sensing with the controllerthe output voltage and current of the solar module. When the controllersenses the solar module is producing power below a power thresholdlevel, the controller sends a signal to the solar array tracker to varythe solar module's pointing until the solar module is operating at itsmaximum power point for the solar module's level of illumination. And,when the controller senses that the maximum power of the solar module isless than a minimum bypass threshold level, the controller sends acommand to the bi-position switch to bypass current around the solarmodule, thereby maximizing solar power array efficiency. In one or moreembodiments, the bi-position switch is a latching solenoid. In someembodiments, the at bi-position switch is a latching relay. In at leastone embodiment, the bi-position switch is a read relay.

In one or more embodiments, the method for providing diodelessterrestrial photovoltaic solar power arrays involves a solar power arraydevice without blocking diodes and without bypass diodes. This methodcomprises providing a solar module; and connecting a solar arraytracker, a circuit breaker, and a bi-position switch to the solarmodule. The method further comprises connecting a power bus and acontroller to the circuit breaker and the bi-position switch. The methodalso comprises connecting an inverter to the power bus, and sensing withthe controller the output voltage and current of the solar module. Whenthe controller senses the solar module is producing power below a powerthreshold level, the controller sends a signal to the solar arraytracker to vary the solar module's pointing until the solar module isoperating at its maximum power point for the solar module's level ofillumination. And, when the controller senses that shadowing on thesolar module is less than a minimum shadowing threshold level, thecontroller sends a command to the circuit breaker to prevent currentflow into the solar module, thereby preventing harm to cells of thesolar module. Also, when the controller senses that the maximum power ofthe solar module is less than a minimum bypass threshold level, thecontroller sends a command to the bi-position switch to bypass currentaround the solar module, thereby maximizing solar power arrayefficiency.

In some embodiments, the device for diodeless terrestrial photovoltaicsolar power arrays involves a solar power array device without blockingdiodes. This device comprises a solar module, a solar array tracker, acircuit breaker, a power bus, a controller, and an inverter. Thecontroller of this device senses output voltage and current of the solarmodule. When the controller senses the solar module is producing powerbelow a power threshold level, the controller sends a signal to thesolar array tracker to vary the solar module's pointing until the solarmodule is operating at its maximum power point for the solar module'slevel of illumination. And, when the controller senses that shadowing onthe solar module is less than a minimum shadowing threshold level, thecontroller sends a command to the circuit breaker to prevent currentflow into the solar module, thereby preventing harm to cells of thesolar module.

DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is an illustration of a model photovoltaic solar power plantshowing no current flowing from the unshadowed string of solar cells tothe shadowed string of solar cells, in accordance with at least oneembodiment of the present disclosure.

FIG. 2 is a graph showing that, for a model solar power array, currentin the shadowed string of solar cells is always greater than zero foroperation below the maximum power point, in accordance with at least oneembodiment of the present disclosure.

FIG. 3 is a depiction of a concentrated photovoltaic (CPV) solar powerfacility without bypass or blocking diodes, in accordance with at leastone embodiment of the present disclosure.

FIG. 4 is an illustration of a conventional solar cell model, whichincludes external diodes for current and voltage output.

FIG. 5 is a depiction of a generalized version of a solar power arrayconsisting of a plurality of solar cells, in accordance with at leastone embodiment of the present disclosure.

FIG. 6 is a graph showing the current and power for a simulated model ofa solar power array, in accordance with at least one embodiment of thepresent disclosure.

FIG. 7A illustrates a solar power array design with solar cell voltagesensing, in accordance with at least one embodiment of the presentdisclosure.

FIG. 7B depicts a solar power array design with module voltage sensing,in accordance with at least one embodiment of the present disclosure.

DESCRIPTION

The methods, devices, and apparatus disclosed herein provide anoperative system for photovoltaic solar power arrays. Specifically, thissystem allows for diodeless terrestrial photovoltaic solar power arrays.The disclosed system is used to monitor the output voltage and currentof a terrestrial solar array. The system sends control signals to atracking system and direct current/alternating current (DC/AC) inverterin order to maximize output power. In addition, if the system senses anabnormal voltage or current condition, the system sends commands toisolate the solar power array's output bus. This system effectivelyprovides functions of bypass and blocking diodes and, in addition,provides intelligence and control for the overall solar power array.

The system of the present disclosure consists of a sensor andcontroller, which controls an array or field of photovoltaic (PV) cellsin such a manner that solar cell bypass diodes and/or blocking diodesare not required. The cost of the solar power array is greatly reducedwith the elimination of blocking diodes and/or bypass diodes. Inaddition, the elimination of bypass diodes and/or blocking diodes from asolar power array design can lead to a slight improvement in powergeneration efficiency and greater solar power array reliability.

Generally, terrestrial solar power arrays use bypass diodes to shuntcurrent around failed cells. And, blocking diodes are typically used byterrestrial solar power arrays to limit the current that can flow into ashadowed array if it is connected in parallel with one or more arraysthat are not shadowed. Shadowing occurs when the level of sunlight thatfalls on one or two of the arrays, in a chain of many arrays, is reducedby clouds. In this situation, the solar cells will still produce power,but at a greatly reduced level. Under these circumstances, current fromthe unshadowed arrays can flow into the shadowed arrays' solar cells andpotentially cause harm to the cells. When current flows to the solarcells in this fashion, the cells act as light emitting diodes (LEDs),and will glow if the current is high enough. This is not normally aproblem at room temperature, but rather is typically a problem at highertemperatures associated with a concentrated photovoltaic (CPV) systembecause this high current can cause damage to the solar cells. Thesystem of the present disclosure prevents “LED current” from flowing toa shadowed array by using a unique property associated with theoperation of multiple arrays connected in parallel.

In one or more embodiments, three features of the disclosed system allowfor the elimination of bypass and/or blocking diodes in terrestrialsolar arrays. These features are: (1) operation of a shadowed array ator near its maximum power point (i.e., maximum current at maximum power(Imp), maximum voltage at maximum power (Vmp)) corresponding to itsreduced level of illumination, thereby preventing current from flowinginto it from other arrays connected in parallel that are not as heavilyshadowed, (2) sensing the array's current and voltage in combinationwith a controller, which can open a circuit breaker automatically when afault condition is detected, thereby protecting the array cells fromharm, and (3) sensor and control functions are able to be integratedinto the array's existing maximum power pointing and trackingelectronics and software. Although the system of the present disclosureallows for the elimination of both types of these diodes, it is notnecessary to eliminate both types of these diodes from the solar powerarray design. If certain situations dictate a need for blocking diodes,bypass diodes may be eliminated, and vice versa.

FIG. 1 depicts a simple solar array model 100 that is used todemonstrate the first feature, which allows for the avoidance ofpotentially harmful light emitting diode (LED) current flow into ashadowed array 110. In particular, FIG. 1 is an illustration of a modelphotovoltaic solar power plant 100 showing no current flowing from theunshadowed string 120 of solar cells (Cell 12, Cell 22) to the shadowedstring 110 of solar cells (Cell 11, Cell 21), in accordance with atleast one embodiment of the present disclosure. In this figure, thephotovoltaic (PV) solar power plant model 100 consists of two parallelstrings 110, 120 of cells (Cell 11, Cell 21, Cell 12, Cell 22), whereeach string of cells includes two solar cells connected in series. Itshould be noted that solar power arrays with more parallel stringsand/or more series cells behave similarly to this model. The cells (Cell11, Cell 21) in the left string 110 are shadowed, and are assumed to bereceiving only 1% of the energy that the second string 120 is receivingfrom the sun. The solar cells (Cell 11, Cell 21, Cell 12, Cell 22)provide power to a load (RL). This model is used to demonstrate that aslong as the shadowed string 110 is operating near its maximum peak powerpoint, current from the unshadowed string 120 will not flow into theshadowed string 110, and potentially harm the shadowed string's cells(Cell 11, Cell 21).

The current-voltage (IV) curves for each string 110, 120, the totalcurrent, and the total power for the array are shown in FIG. 2.Specifically, FIG. 2 is a graph showing that, for a model solar powerarray 100, current in the shadowed string 110 of solar cells is alwaysgreater than zero for operation below the maximum power point. In thisfigure, the unshadowed string 120 (I2 on graph) is shown to produce 7.1Amps, while the current produced by the shadowed string 110 (I1 ongraph) is 71 milli Amps (mA) below the maximum power point (PT ongraph). And, the total current for the system is shown in curve IT onthe graph. In this situation, the shadowed string 110 is notsubstantially contributing to the power production, and could beeliminated from the circuit without impacting the total power outputsignificantly (1% was chosen arbitrarily, and the real percentage couldbe higher or lower). It should be noted that the current in the shadowedstring 110 (I1 on graph)) remains positive to the left of the maximumpower point.

If the operating point shifts to the right of the maximum power point,then current flow reverses direction and current from the unshadowedstring 120 flows through to the cells (Cell 11, Cell 21) in the oppositedirection. If the current is large enough, the cells (Cell 11, Cell 21)act as light emitting diodes (LEDs) and will create photons. The amountof current flowing through the shadowed string 110 in the LED directiondepends on cell characteristics, and may or may not cause cell damage.In any event, one of the main purposes of the present system is toprevent this LED current situation from occurring by ensuring that thetotal solar power array system is operating at or near its maximum powerpoint.

The system of the present disclosure senses the current through theshadowed string 110 along with its voltage, and determines if the powerbeing produced by the string 110 is below a threshold level. If thepower is above the threshold level, then the controller does nothing. Ifthe power output is below the threshold level (indicating that LEDcurrent may be imminent), the controller commands the array's trackingsystem to find the pointing direction that maximizes the string's outputpower. It should be noted that this not necessary if the solar powerarray already includes a maximum power tracker. If the power remainsbelow the threshold level, the controller communicates with an inverter,and commands it to increase its load (i.e., lower its internalresistance), which increases current flow from the shadowed string 110.However, if the inverter shuts down, or is not operating at its maximumpower point, the controller will open the shadowed string's 110 circuitbreaker until the controller senses that the string 110 is no longershadowed. The controller can determine if the string 110 is no longershadowed by sensing the string's 110 voltage, which is an indicator ofsolar illumination.

The second feature, which allows for the elimination of blocking diodes,will now be discussed in detail. If an abnormal current or voltage issensed by the solar power array exceeding a threshold level, which maybe either positive or negative, the controller commands an array circuitbreaker to open, thereby preventing any damage to the array. The circuitbreaker can be reset manually or automatically, which may depend on thesolar power plant's operating procedure. In one or more embodiments, thecircuit breaker may be replaced by a properly sized fuse, if systemconsiderations show that this is a more economical approach. If afailure mode exists that cannot be avoided with this implementation,then it may be cost effective to include a few spare arrays in the powersystem to replace the few that might be damaged by a rare event, therebystill avoiding the need for employing blocking diodes.

The third feature, which involves integration of the disclosed systeminto existing array electronics and software, will now be discussed indetail. Current array tracker systems include electronics that sensearray bus voltage and current flow. This sensed data is used to pointthe array with a solar tracker. A central processing unit (CPU) is usedto process this data and issue the related commands. In one or moreembodiments, the system of the present disclosure requires that asoftware module be included that recognizes when current through anarray is beginning to reverse due to shadowing, and issues commands toensure that the solar tracker and/or DC/AC inverter maintain operationat the solar power array's maximum power point. In addition, in at leastone embodiment, the disclosed system also requires software to analyzethe current and voltage data in order to identify abnormal busconditions, and to command the circuit breaker to open in a case when afault is detected.

The elimination of blocking diodes in the present system is possiblecause of the unique characteristics of strings of arrays connected inparallel that are operating at or near their maximum operating powerpoint. That is, by maintaining operation near this point, current fromfully illuminated arrays (strings) will not flow into shadowed arrays(strings) and potentially damage the shadowed cells.

FIG. 3 is a depiction of a concentrated photovoltaic (CPV) solar powerfacility 300 without bypass or blocking diodes, in accordance with atleast one embodiment of the present disclosure. In particular, FIG. 3shows a block diagram of a solar power element containing P arrays 310,312, 314 connected in parallel that are connected to a DC/AC inverter320, which is feeding power to a grid 330 (commercial power lines orcombined with other inverter outputs). In this figure, the array 310,312, 314 solar cells do not include bypass diodes, and the blockingdiodes are replaced by a circuit breaker (CB) 340, 342, 344 (or a fusein the simplest case) that is operated by a controller 350, 352, 354.Each controller 350, 352, 354 senses its own array's 310, 312, 314output voltage and current (i.e., I1 and V1 in the first array 310; I2and V2 in the second array 312; IP and VP in the P array 314). If thearray 310, 312, 314 is producing power below a given threshold level, itsends a signal to the tracker to vary the array's 310, 312, 314 pointingslightly until the array 310, 312, 314 is operating at the maximum powerpoint for the array's 310, 312, 314 level of illumination. It should benoted that this procedure is the same operation used for the maximumpower tracker function. If shadowing on the array 310, 312, 314 is lessthan a minimum threshold level, the controller 350, 352, 354 sends acommand to the CB 340, 342, 344 to open the connection to the mainpositive power bus, thereby preventing current from flowing into thearray 310, 312, 314 and harming the cells.

When the array's 310, 312, 314 open circuit voltage exceeds a thresholdlevel, this indicates that the array 310, 312, 314 is no longershadowed, and the controller 350, 352, 354 sends a command to the CB340, 342, 344 to close the circuit. The controller 350, 352, 354 thenoptimizes the array's 310, 312, 314 power operating point until the nextshadowing event occurs. An optional connection between the controllers350, 352, 354 and inverter 320 is shown because the maximum power pointcan also be controlled by the inverter 320, thereby eliminating the needfor each controller 350, 352, 354 to iterate its array 310, 312, 314pointing to locate this power point. If this is a normal operating modeof the inverter 320, this optional connection is not necessary.

If a ground fault or other circuit anomaly occurs, the controller 350,352, 354 senses it, and opens the circuit breaker 340, 342, 344. Thecircuit breaker 340, 342, 344 can also operate autonomously in the eventof an anomaly, and will notify the controller 350, 352, 354 of thisaction. A fuse is shown as an alternative to the CB 340, 342, 344 forfault isolation if the solar cells can withstand a reverse LED currentthat is above the fuse flash point. With fuses implemented in thesystem, in the case of a fault, the fuse would have to be replaced, andthe cause of the fault would have to be eliminated. The controller 350,352, 354 will also sense an open circuit failure, and will disconnectthe array 310, 312, 314 from the bus since the array 310, 312, 314 willnot produce any power in this state. The source of the failure can thenbe identified and repaired during routine maintenance.

In one or more embodiments, a solar array typically comprises a numberof solar cells that are connected in series. A plurality of solar cellsthat are grouped together are often referred to as a “module.” FIG. 4 isan illustration of a conventional solar cell model 400, which includesexternal diodes 410, 420 for current and voltage output. The model forthe current-voltage relationship of a single solar cell 400, which isshown in FIG. 4, with an illumination factor of S (0≦S≦1) is given bythe following relationship:

$\begin{matrix}{I_{cell} = {{S\; I_{sc}} - {I_{o}\{ {{\exp \lbrack \frac{q( {V_{Load} - {\Delta \; V} + {I_{cell}R_{s}}} )}{m\; k\; T} \rbrack} - 1} \}} - \frac{V_{Load} - {\Delta \; V} + {I_{cell}R_{s}}}{R_{sh}}}} & ( {2\text{-}1} )\end{matrix}$

where,

ΔV=(1−S)I _(sc) R _(s)

V=V _(Load) +I _(sc) R _(s)

I_(o)=dark currentq=electron chargem=cell ideality factork=Boltzmann's constantV=voltage across cell diodeV_(Load) load voltageS=shadowing factorI_(sc)=short current produced by the cellR_(s)=cell's series resistanceR_(sh)=cell's shunt resistance.

The ΔV term describes the shift in the current-voltage (IV) curve tohigher values of voltage that occurs as the light intensity decreases.The voltage term V is the bias voltage across the solar cell's 400photovoltaic junction that controls the current flow through the solarcell 400.

The single solar cell model 400 is also applicable to solar cells 400with multiple junctions if the proper model parameters are chosen (e.g.,for a three junction cell, the ideality factor is about 3). Thedirection of the current I_(cell)(I_(L)) 430 is out of the device intothe positive side of the load (RL) 440. Electronically, the solar cell400 is equivalent to a current generator connected in parallel with anasymmetric, non-linear resistive element, i.e., a diode. Whenilluminated, the solar cell 400 produces a photocurrent proportional tothe light intensity. The photocurrent is divided between the variableresistance of the diode 450 and the load 440, in a ratio that depends onthe resistance of the load 440 and the intensity of the illumination.For higher resistances, more of the photocurrent flows through the diode450, resulting in a higher potential difference between the solar cell400 terminals, but a smaller current through the load 440. The diode 450provides the “photovoltage.” Without the diode 450, there is nothing todrive the current through the load 440 (i.e., no potential differenceoccurs across the load 440). The series 460 and shunt 470 resistancesaffect the shape of the solar cell's 400 IV characteristic.

If the solar cell 400 is connected to a variable voltage source, theload voltage can be increased or its polarity inverted. If the voltageis increased high enough, the solar cell 400 ceases to function as asolar cell 400, and instead operates as a light emitting diode (LED). Ifthe voltage across the solar cell 400 is negative, the solar cell 400acts as a photo detector, consuming power to generate photocurrent,which is light dependent, but independent of the voltage across it.These effects become important when a solar cell 400 or solar cells 400become shadowed, or when arrays are connected in parallel with otherarrays of varying voltage outputs.

FIG. 5 is a depiction of a generalized version of a solar power array500 consisting of a plurality of solar cells, in accordance with atleast one embodiment of the present disclosure. A mathematical model ofthe solar power array 500 in FIG. 5 can be constructed based on thesimple model of Eq. 2-1. As such, the equations describing a fullyilluminated field composed of P strings containing N cells is given by,

$\begin{matrix}{{{{I\_}1} + {{I\_}2} + {\ldots \mspace{14mu} {I\_ P}}} = {\frac{V_{Load}}{R_{L}}.}} & ( {2\text{-}2} )\end{matrix}$

The current in each of the fully illuminated strings with N functioningcells with identical characteristics is given by,

$\begin{matrix}{I = {I_{sc} - {I_{o}\{ {{\exp\lbrack \frac{q( {\frac{V_{Load} - ( V_{od} )}{N} + {IR}_{s}} )}{m\; k\; T} \rbrack} - 1} \}} - \frac{( {\frac{V_{Load} + ( V_{od} )}{N} + {IR}_{s}} )}{R_{sh}}}} & ( {2\text{-}3} )\end{matrix}$

where,I=string currentV_(od)=blocking diode voltage dropN=number of cells in a string.

Numerical methods can be used to solve Eq. 2-2 using Eq. 2-3 for each ofthe current terms respectively. For this calculation, an additional termrepresenting the bypass diode voltage (where power dissipation isproportional to current as is usually the case for large current diodesat direct current (DC)) may be added to the load voltage.

FIG. 6 is a graph showing the current and power for a simulated model ofa solar power array, in accordance with at least one embodiment of thepresent disclosure. In particular, FIG. 6 shows an exemplarycurrent-voltage (IV) curve for a solar power array device modelcomprising 71 solar arrays connected in parallel, with each arrayincluding 144 solar cells connected in series. The IT curve shows thecurrent through the load, and the PT curve shows the power delivered tothe load. Peak power production corresponds to a specific load currentand load voltage. The peak block power output is approximately 230,853Watts (W) or 3,251.5 Watts (W) per array. This is the point of operationthat the disclosed system will be operating (this point is setautomatically by DC/AC inverter).

The model results are better than a practical implementation because ofthe absence of the series and shunt resistances. As a result, themodel's form factor is about 86.4% and the power output is higher than areal array. However, this is a close enough approximation to demonstratethe theory underlying the elimination of bypass diodes.

The first step in the bypass diode elimination process is to identify ameans to detect which solar cells in an array have failed (as with anopen circuit situation). Without bypass diodes, one or more failed solarcells will block current from flowing into the load. Thus, sensing theabsence of current identifies an array with one or more failed cells.Even if an array is heavily shaded, some finite current still flows, sothe failure detection logic is still valid. Detection of a specificfailed solar cell is more complicated, and requires sensing the voltageacross each cell after a failure is detected, and comparing it with aknown value. It can be shown that the voltage across a failed cell (itis assumed all cells are essentially identical) is given by,

BV _(By) +LV _(oc) −V _(Load)=0.  (2-3)

where,

B=number of failed cells in the array

V_(By)=voltage across a failed cell

L=number of operating cells in the array (2-4)

V_(oc)=cell open circuit voltage

V_(Load)=voltage across the load.

The total number of cells in the array is K such that,

K=B+L.  (2-5)

From Eq. 2-3, the voltage across a failed cell is,

$\begin{matrix}{V_{By} = {\frac{V_{Load} - {L\; V_{oc}}}{B}.}} & ( {2\text{-}6} )\end{matrix}$

Substituting Eq. 2-5 into (2-6) yields,

$\begin{matrix}{\begin{matrix}{V_{By} = \frac{V_{Load} - {( {K - B} )V_{oc}}}{B}} \\{= {\frac{V_{Load}}{B} - {\frac{K}{B}V_{oc}} + V_{oc}}}\end{matrix}{{{\frac{K}{B}( {\frac{V_{Load}}{K} - V_{oc}} )} + V_{oc}} = {{\frac{{KV}_{oc}}{B}( {\frac{V_{Load}}{K\; V_{oc}} - 1} )} + {V_{oc}.}}}} & ( {{2\text{-}7\mspace{14mu} a},b,c,d} )\end{matrix}$

The term

$\frac{V_{Load}}{K\; V_{oc}}$

is recognized as the ratio of the maximum power load voltage to theoptimum power load voltage that is used in the Form Factor equation. Forthe 71 array power block shown in FIG. 5, this ratio is about 0.9 underfull illumination. It is smaller for actual arrays due to the effects ofthe series and shunt resistances. Using this value for the term, it isseen that,

$\begin{matrix}{V_{By} = {{{\frac{K\; V_{oc}}{B}( {0.9 - 1} )} + V_{oc}} = {( {1 - {0.1\frac{K}{B}}} ){V_{oc}.}}}} & ( {2\text{-}8} )\end{matrix}$

This equation shows that V_(By) will be

${\langle{0\mspace{14mu} {for}\mspace{14mu} \frac{K}{B}}\rangle}10.$

For example, if one solar cell ((B=1) out of 144 cells fails, then

${0.1\frac{K}{B}} = {14.4.}$

The corresponding voltage is −13.4 V. If two cells fail (B=2), then theterm is 7.2 and the voltage is −0.29 V. In summary, the voltage acrossthe failed cell will be negative for up to 14 cell failures. In fact, ifa failure threshold is set on V_(By) at

$\frac{V_{oc}}{2}$

it can be shown that more than 28 cells will have to fail before thethreshold is exceeded, and the test would fail. It is clear that ifindividual solar cell voltages are sensed and found to be less than zero(i.e., negative) or below a specified threshold, and this fact iscombined with the knowledge that no current is being produced in thearray, then the solar cell is a failed (open circuited) solar cell.

If more than one cell fails in a single array over a 25-year life, thenthe design is faulty. The value of V_(By) is used to determine if cellfailure is not an issue. The fact that the ratio

$\frac{V_{Load}}{K\; V_{oc}}$

for real systems is smaller the ideal value used here means that thenumber of cell failures yielding a negative cell voltage is much higherthan 14, so this is not a limitation of the approach. More generally,Eq. 2-5 can be used to further extend the failed cell identificationmethod to specific threshold voltages (non-negative) provided that theload voltage is measured as well since the other parameters are known apriori and deal with even more cell failures.

In comparison, the voltage across a fully functioning illuminated cellproducing peak current is given by the equation,

$\begin{matrix}{V_{cell} = {\frac{m\; k\; T_{cell}}{q}{{\ln ( {\frac{I_{sc} - I_{cell} + I_{sh}}{I_{o}} + 1} )}.}}} & ( {2\text{-}9} )\end{matrix}$

For the 71 array block, the individual solar cell voltage in the 70functioning arrays is 3.254 V at the maximum power point, where the opencircuit voltage is 3.572 V (I_(sc) is 7.1309 A and I_(o) is 1.7×10⁻¹⁷A). It is unlikely that a failed cell will be misidentified because ofthe large positive open circuit voltage of functioning cells in a nocurrent state.

A similar situation occurs when a cell or group of cells is shaded andthe photocurrent is reduced relative to the remaining fully functioningcells. Assuming that the array is operated near its peak power point,their voltage drops to near zero (since the load current exceeds theshort circuit current for the shaded condition), or they become reversedbiased. This limits the available current through the string if bypassdiodes were not present to shunt current around the cell. However, sincethe solar cell is reversed biased, it is not producing power and itsphotocurrent simply reduces the amount of current through the shunt orbypass diode. Thus, these diodes act as a power consuming shunt acrossthe shadowed solar cells.

In one or more embodiments, a failed solar cell or shadowed solar cellcan be identified if both the current through an array and the voltageacross each solar cell (or each module) in an array is known. If thecurrent is zero and the voltage is negative across a solar cell, thesolar cell is failed in an open circuit mode. A module with one or moresolar cell failures can be treated the same way, but with a differentvoltage threshold level for a failure. Once a solar cell failure isdetected, the required response is to create a low resistance shuntaround the failed solar cell. This eliminates the power loss associatedwith a bypass diode.

If a solar cell is merely shaded and has not failed, its voltage will beless than its normal open circuit voltage, and the current in the arraywill be less than its peak power value. Thus, current measurements canbe used to determine that one or more cells are being shadowed. Ifbypass diodes are not present across each cell, the voltage across theunshaded cells will also be reduced so that voltage measurements cannotbe used to determine which cells are limiting array current. However,the shaded cells can be identified by trial and error using the processdescribed in the following section.

In the case of a solar module, relatively more power will be lost ifthere are still functioning solar cells in the bypassed modulecontaining the failed solar cell. However, this loss is relatively minor(˜20 W), and will not affect the overall solar power plant output powersignificantly.

Two implementations of the diodeless concept are shown in FIGS. 7A and7B that allow bypass diodes to be eliminated from the solar power array.Specifically, FIG. 7A illustrates a solar power array design 700 withsolar cell voltage sensing, and FIG. 7B depicts a solar power arraydesign 750 with module voltage sensing, in accordance with at least oneembodiment of the present disclosure.

Both concepts also include a controllable circuit breaker 705, 755 atthe solar power array's output to the inverter, and/or a fuse 710, 760that are needed to eliminate the blocking diode. The concept implementedat the solar cell level is shown in FIG. 7A. It can be seen in thisfigure that the voltage across each solar cell is sensed by thecontroller 705 through the cell voltage sense lines. The maximum powertracker 715 already senses array current using the current sensor 720.Logic is included in the maximum power tracker's 715 software todetermine if a failure has occurred, and which solar cell or solar cellshave failed. The maximum power tracker then commands the bi-positionswitch 725 (e.g., a single-pole double-throw switch) to shunt currentaround each failed solar cell. Types of bi-position switches that may beused for the disclosed system include, but are not limited to, latchingsolenoids, latching relays, and read relays. Any device that willproduce a short circuit across the failed solar cell with a small pulseof power, and then stay latched into that state without further powermay be employed for the bi-position switches of this system. This deviceshould also be capable of opening as well after being energized with asmall burst of power, and then remain in the open state.

Another option for providing a shunt around a failed solar cell is toactivate a latching solenoid using the current drawn from the operatingarray using a normally reversed biased diode across the solar cell,where the latching solenoid coils when the cell opens and will stopcoiling when the latch closes the bypass shunt. However, the currentdraw of this device must also be small enough that it does not reducethe open circuit voltage in the failed array below the voltage of thecells in the fully illuminating arrays. If this occurs, then no currentwill flow though the device and this implementation may not bereversible.

FIG. 7B shows the concept implemented at the module level. Thisparticular implementation reduces the number of voltage sense linesrequired and parts count at the expense of a slightly greater power lossif a module contains one or more failed solar cells. For the model solarpower array, this means sensing voltage from 24 modules instead of 144individual solar cells, which may be more practical since the voltagesense lines and bi-position switches can be located outside the module.

A potential simplification of the concept may be possible if the unitdata “personalities” of each solar cell in a solar power array can becharacterized, and remain stable over the life of the array. In thiscase, it may be possible to use only the array voltage and current toidentify which of the solar cells (or modules) have failed since eachsolar cell (or module) will affect the solar power array's current andvoltage in a unique fashion. For this operation, the maximum powertracker sequences through all possible solar cell failure states untilthe solar power array begins to function properly. This approach also isapplicable to the identification of shaded solar cells. When the rightcombination of solar cells is shunted, solar power array current willexperience a significant increase, thereby indicating that the shadedsolar cells have been bypassed.

Having eliminated the need for bypass diodes and their power loss, it ispossible to consider including a current-voltage (IV) curve measurementcapability in the maximum power tracker for each solar cell or module inaddition to a voltage sensor. With this information, a strategy tomaximize the solar power array's overall power output can beimplemented, and this will help to mitigate much of the problemsassociated with solar cell-to-cell and module-to-module variability. Asimple strategy would be to short circuit a poorly performing solar cellthat is limiting the remaining 143 solar cells in a model array fromperforming to their maximum potential. This may be required as solarcells degrade over the system's life. Another option is to have themaximum power tracker control the voltage across a solar cell or groupof solar cells to produce a desired current output for each solar powerarray. In doing so, each solar power array could be maintained at itsmaximum output power point, and the inverter would not have to performthis function.

Although certain illustrative embodiments and methods have beendisclosed herein, it can be apparent from the foregoing disclosure tothose skilled in the art that variations and modifications of suchembodiments and methods can be made without departing from the truespirit and scope of the art disclosed. Many other examples of the artdisclosed exist, each differing from others in matters of detail only.Accordingly, it is intended that the art disclosed shall be limited onlyto the extent required by the appended claims and the rules andprinciples of applicable law.

1. A method for providing a solar power array device without blockingdiodes, the method comprising: providing a solar module; connecting acircuit breaker to the solar module; connecting a power bus and acontroller to the circuit breaker; connecting an inverter to the powerbus; sensing with the controller the output voltage and current of thesolar module; and determining, with the controller, an amount ofshadowing on the solar module by using the output voltage and the outputcurrent of the solar module, wherein when the controller determines thatthe amount shadowing on the solar module is less than a minimumshadowing threshold level, the controller sends a command to the circuitbreaker to prevent current flow into the solar module.
 2. The method ofclaim 1, wherein the inverter is a direct current/alternating current(DC/AC) inverter.
 3. The method of claim 1, wherein the solar modulecontains a plurality of solar cells.
 4. The method of claim 3, whereinthe plurality of solar cells are connected in series.
 5. The method ofclaim 3, wherein the plurality of solar cells are connected in parallel.6. The method of claim 3, wherein the plurality of solar cells areconnected in parallel-series combination.
 7. The method of claim 1,wherein the solar module contains only one solar cell.
 8. A method forproviding a solar power array device without bypass diodes, the methodcomprising: providing a solar module; connecting a bi-position switch tothe solar module; connecting a power bus and a controller to thebi-position switch; connecting an inverter to the power bus; and sensingwith the controller the output voltage and current of the solar module,wherein when the controller senses that the maximum power of the solarmodule is less than a minimum bypass threshold level, the controllersends a command to the bi-position switch to bypass current around thesolar module.
 9. The method of claim 8, wherein the inverter is a directcurrent/alternating current (DC/AC) inverter.
 10. The method of claim 8,wherein the solar module contains a plurality of solar cells.
 11. Themethod of claim 10, wherein the plurality of solar cells are connectedin series.
 12. The method of claim 10, wherein the plurality of solarcells are connected in parallel.
 13. The method of claim 10, wherein theplurality of solar cells are connected in parallel-series combination.14. The method of claim 8, wherein the solar module contains only onesolar cell.
 15. The method of claim 8, wherein the bi-position switch isa latching solenoid.
 16. The method of claim 8, wherein the atbi-position switch is a latching relay.
 17. The method of claim 8,wherein the bi-position switch is a read relay.
 18. A method forproviding a solar power array device without blocking diodes and bypassdiodes, the method comprising: providing a solar module; connecting acircuit breaker and a bi-position switch to the solar module; connectinga power bus and a controller to the circuit breaker and the bi-positionswitch; connecting an inverter to the power bus; sensing with thecontroller the output voltage and current of the solar module, whereinwhen the controller senses that the maximum power of the solar module isless than a minimum bypass threshold level, the controller sends acommand to the bi-position switch to bypass current around the solarmodule; and determining, with the controller, an amount of shadowing ofthe solar module by using the output voltage and the output current ofthe solar module, wherein when the controller determines that the amountof shadowing on the solar module is less than a minimum shadowingthreshold level, the controller sends a command to the circuit breakerto prevent current flow into the solar module.
 19. The method of claim18, wherein the inverter is a direct current/alternating current (DC/AC)inverter.
 20. A solar power array device without blocking diodes,comprising: a solar module; a circuit breaker; a power bus; acontroller; and an inverter, wherein the controller senses outputvoltage and current of the solar module, wherein the controllerdetermines an amount of shadowing on the solar module by using theoutput voltage and the output current of the solar module, when thecontroller determines that the amount of shadowing on the solar moduleis less than a minimum shadowing threshold level, the controller sends acommand to the circuit breaker to prevent current flow into the solarmodule.